Structural and physical properties of the chiral antiferromagnet CeRhC2

We report a study of the structural, magnetic, transport, and thermodynamic properties of polycrystalline samples of CeRhC2 and LaRhC2. CeRhC2 crystallizes in a tetragonal structure with space group P41, and it orders antiferromagnetically below TN1≈1.8K. Powder neutron diffraction measurements reveal a chiral magnetic structure with a single propagation vector Qm=(1/2,1/2,0.228(5)), indicating an antiferromagnetic arrangement of Ce magnetic moments in the ab plane and incommensurate order along the c axis with a root-mean-square ordered moment of mord=0.68μB. A second antiferromagnetic phase (TN2) becomes apparent in electrical resistivity, Hall, and heat capacity measurements in fields above 0.3 T. Electrical resistivity measurements reveal that LaRhC2 is a semiconductor with a band gap of Eg∼24 meV, whereas resistivity and Hall measurements indicate that CeRhC2 is a semimetal with a carrier concentration of n∼1020cm-3. With applied hydrostatic pressure, the zero-field antiferromagnetic transition of CeRhC2 is slightly enhanced, and CeRhC2 becomes notably more metallic up to 1.36 GPa. The trend toward metallicity is in line with density-functional calculations that indicate that both LaRhC2 and CeRhC2 are semimetals, but the band overlap is larger for CeRhC2, which has a smaller unit cell volume than that of its La counterpart.